A. Field of the Invention
The present invention relates to a method for producing food grade hydrated lime, Ca(OH)2, which meets the standards as required by CODEX without requiring extensive changes in existing equipment or process steps.
B. Description of the Prior Art
Lime has a variety of uses. It is commonly used in treating waste water and sewage. It is used in agriculture to neutralize acidic soils and to provide nutrients for sustaining plant life. Lime is also used extensively in construction for the stabilization of soils and as a component in a variety of building materials. Lime is also used in a variety of “food grade” products intended for human consumption.
In this description, the term “lime” will be understood to mean both quicklime (calcium oxide) and hydrated lime (calcium hydroxide). Quicklime is produced by heating limestone (calcium carbonate) in a kiln at extreme temperatures to “calcine” the material and thereby drive off carbon dioxide. Quicklime is usually in the form of lumps or pebbles. In order to further process lime and improve the ease with which it is handled, dry lime is often mixed with water to form a slurry. In the case of quicklime, the water reacts with the quicklime in an exothermic reaction to form hydrated lime. This is often referred to as slaking. During the slaking of quicklime, large amounts of heat are given off which can significantly raise the temperature of the slurry. Water can then be driven off to produce dry, hydrated lime which is usually a powder.
Food grade hydrated lime is a specific material that is sold to the food processing industry in the United States. The specifications for food grade hydrated lime are defined by CODEX. The CODEX Alimentarius Commission was created in 1962 by two U.N. organizations, the Food and Agricultural Organization (FAO) and the World Health Organization (WHO). CODEX is the major international mechanism for encouraging fair international trade in food while promoting the health and economic interests of consumers. Within the United States, CODEX activities are coordinated by officials from the U.S. Department of Agriculture, the U.S. Food and Drug Administration and the U.S. Environmental Protection Agency. In the United States, there are a number of companies that produce “normal”, i.e., industrial grade, hydrated lime for industrial use. However, there are only two companies known to Applicant at the present time that produce “food grade” hydrated lime. This is due, at least in part, to the exacting chemical specifications required by CODEX. Many hydrated limes that are suitable for general industrial use, fail to meet the CODEX standards since they exceed, for example, the limits for trace metals found in the compositions.
As mentioned above, hydrated lime is produced by first heating limestone in a kiln (calciner) to remove carbon dioxide and form quicklime. To improve the handling characteristics of the quicklime, a controlled amount of water is then added to form calcium hydroxide, commonly referred to as hydrated lime. To achieve the CODEX chemical specifications requires the use of the purest limestone as calciner feed and normally the use of natural gas as a fuel for the kiln. The use of natural gas, even though it is more expensive, is preferred over solid fuel such as coal or coke, in that solid fuel tends to contaminate the quicklime and thus the hydrated lime made from the quicklime with combustion ash.
A need exists for a method for producing food grade hydrated lime which meets CODEX specifications without drastic changes in the equipment or process parameters presently employed in calcining limestone to produce quicklime, or in slaking the quicklime to produce hydrated lime.
A need exists for a method for producing food grade hydrated lime which would allow the use of normal limestone as feed to the calciner, without requiring the purest of limestone as feed.
A need also exists for such a process which would allow the use of solid fuel sources in the calciner, rather than requiring the use of more expensive natural gas as a fuel source.